Etiopathology

ARN is caused by the members of Herpesviridae family, including herpes simplex virus (HSV) types 1 and 2 and varicella zoster virus (VZV). Primary infections by HSV and VZV are common in childhood, and these become latent in ganglion neurons. Although latent infections caused by these viruses are common, the precise mechanism underlying ARN development in healthy adults remains unclear. Some loci of the human leukocyte antigen class II have been associated with the development of ARN (Holland et al. 1989). Rochat et al. revealed some immune imbalance in ARN patients (Rochat et al. 1996), indicating that immune deficiency may be a cause for the onset of the disease. On the other hand, PORN is observed in immunocompromised host such as patients with AIDS, patients receiving immunosuppressive agents for autoimmune diseases or after organ transplantation, and patients receiving chemotherapy for malignant disease.

Several techniques to diagnose the viral causes of ARN, such as antibody-based analysis of intraocular fluid or serum and viral culture, are available. In an electron microscopic analysis of retinal specimens from patients with ARN and PORN, viral particles morphologically consistent with VZV were detected in all layers of the affected retina (Duker and Blumenkranz 1991; Forster et al. 1990).

Of late, a PCR-based system has become available in the clinic for the detection of the genomes of various pathogens in a small volume of intraocular fluid. The sensitivity and specificity of this system for the detection of VZV and HSV in patients with herpetic retinitis are >95% (Wong et al. 2013). The Japanese ARN Study Group collated the clinical data of ARN patients with HSV-1, HSV-2, or VZV infection confirmed by the PCR system and has advocated a new diagnostic criterion of ARN (Takase et al. 2015). Moreover, quantitative PCR can be used to monitor the disease activity and evaluate the outcomes of treatment for ARN and PORN (Asano et al. 2004; Yin et al. 2007).

Clinical Features

Acute retinal necrosis (ARN): Patients with ARN initially complain of unilateral eye redness, ophthalmalgia, blurred vision, and/or vision loss. Ophthalmological examinations reveal acute unilateral iridocyclitis with granulomatous or fine keratic precipitates and, occasionally, folds in Descemet’s membrane and/or posterior iris synechia in the anterior segment. The intraocular pressure is increased in 30% patients, although it returns to normal with a decrease in inflammation. In the posterior segment, vitreous opacity and multifocal, patchy, yellowish white lesions caused by inflammatory responses against VZV are observed in the peripheral retina (Fig. 27.1). With rapid progression of the disorder in the absence of antiviral therapy, the patchy lesions fuse and extend concentrically with occlusive periarteritis and necrotizing retinitis (Fig. 27.2). In the late stages of ARN, the necrotizing lesions in the retina become thin and atrophy. Then, vitreous traction and contractile membranes may be observed on the surface of the affected retina, leading to multifocal retinal breaks and subsequent RRD within a few months (Fig. 27.3).

In most cases, the optic nerve may be hyperemic and swollen because of optic neuropathy in the early stages (Fig. 27.4), followed by atrophy in the late stages. Occlusion of the retinal arteries around the optic nerve is often observed. The extent of optic nerve damage is also considered as one of the most pivotal indicators of the prognosis of visual acuity (Iwahashi-Shima et al. 2013).

Finally, ARN can develop in the contralateral eye, usually within 6 weeks of onset in the affected eye, in approximately 30% patients; this can be prevented by timely antiviral therapy.

Progressive outer retinal necrosis (PORN): PORN is observed in immunocompromised hosts, such as patients with AIDS (CD4+ T-lymphocyte count ≤ 50/μL) and patients with other conditions associated with immunosuppression. Symptoms include rapid and painless loss of central vision, floaters, and loss of peripheral vision. Ophthalmological examinations initially reveal multifocal, opaque, yellowish white lesions without granular borders, sometimes including areas of confluent opacifications, in the outer layer of the retina in the posterior pole or the periphery (Fig. 27.5) (Engstrom et al. 1994). There is no vasculitis, with minimal or no intraocular inflammation. However, vitritis and vasculitis have been reported to develop with an increase in the CD4+ T-lymphocyte count in patients with AIDS (Gore et al. 2012). Subsequently, the lesions rapidly spread through all retinal layers and progress in the macula, often leading to multifocal retinal breaks and subsequent RRD within several weeks. The visual outcomes are extremely poor despite available treatment options.

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